There are quite a few prior art rock breaking devices. The most basic device is a sledge hammer. An experienced “sledge hammer” operator can only achieve blow energies of around 220 joules in the horizontal plane with the associated fatigue and risk of back injury. Also, hand held pneumatic rock breakers are known, the largest of which provide blow energies of around 100 joules. Because of the recoil, hand held pneumatic rock breakers can only effectively be used vertically down.
Other pneumatic tools are known, such as the percussion tool utilizing negative pressure as disclosed in EP0181486 A (Landmark West Ltd). In this tool a pressure imbalance between a low pressure chamber and a middle chamber creates the force required to accelerate a piston towards a moil. The means by which the impetus is given to the piston is essentially unchanged from a simple pneumatic jackhammer. The force applied to the piston occurs over a short distance of travel, say less than 500 mm, which results in significant reaction forces acting on the tool body and which must be opposed by gravity and by the operator. Furthermore, due to the relatively small size of the low pressure chamber in which a vacuum is created, a significant pressure fluctuation occurs that results in a substantially varied force to the piston. As such the force present in such a tool will vary according to the pressure within the low pressure chamber causing vibration which is undesirable.
Many other pneumatic impact tools are unsuitable for high energy blows. One such pneumatic impact device with recoil damping is disclosed in EP1690647 A (Thyssenkrupp Drauz Nothelfer Gmbh). This device which is used for minimizing vibration on a robot arm during a riveting operation is only suitable for low energy blows. This is particularly evident from the small size of the contact flange 18 shown in FIG. 2. Furthermore the recoil damping in this device occurs after the blow.
Much larger pneumatic recoilless hammers are known, such as the liner bolt removal tool disclosed in International Patent Publication No. WO 2002/081152. This device delivers a 450 joule recoilless blow and weighs 250 kg. In use, it is suspended from above and is manually manipulated. The recoil normally associated with accelerating the hammer piston to strike velocity is absorbed by a much larger free floating mass. This mass is decelerated inside the hammer casing at a rate that is less than the applied force of the operator. Even larger, hydraulic recoilless hammers are known. These hydraulic hammers deliver up to 1500 joule recoilless blows and weigh up to 500 kg, and they are also suspended from above and manually manipulated. The recoil normally associated with accelerating the hammer piston up to strike velocity is absorbed, as in the pneumatic hammer, by a larger mass. In this hammer the larger mass is accelerated forward at a controlled rate prior to the hammer firing. This absorbs the piston acceleration force over a shorter distance.
A disadvantage of the prior art is that the much larger devices that provide blow energies of greater than 150 Joules are considerably heavier than hand held devices. The present invention seeks to provide an alternative recoilless hammer, which can provide blow energies substantially greater than the prior art pneumatic hand held devices, but without the considerable weight disadvantage of the much larger prior art devices.
Within this specification blow energies of less than 150 Joules are considered to be “low blow energies”, and blow energies substantially greater than 150 Joules are considered to be “high blow energies”.